Merge: Ropes integration

Reroll for the integration of Ropes in the new API.

Now that the parent relation is gone in RopeNode, subropes may be shared, this simplifies the code and also improves the overall performance of the structure.

The names have been simplified too, I got tired of writing "ForwardDFSRopeCharIterator".

Pull-Request: #480
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>